An optical receiver detects an optical signal and generates an electrical output signal therefrom having a predetermined amplitude.
The optical signal has a fixed wavelength that is generated by an optical source, such as a laser or light emitting diode (LED). Information is transmitted on the optical signal by modulating the intensity of the optical signal in a detectable way. To transmit large amounts of information per unit time, the optical signal is modulated at a high rate. The higher the modulation rate, the greater the modulation bandwidth of the transmitted optical signal. In designing an optical transmission system, the system components, e.g., regenerators and receivers, must be capable of providing an electrical output signal having a predetermined amplitude over the modulation bandwidth. In addition, an optical receiver in telecommunications applications must provide the electrical output signal with low noise and distortion for a wide range of incoming optical power levels. This additional requirement is reflected in two circuit parameters of optical receivers: dynamic range and sensitivity. Dynamic range is the range of optical signals, i.e., the difference between the maximum optical power level before overloading and the minimum optical power, over which an optical receiver can provide the desired electrical output signal with an acceptable error rate. Sensitivity, on the other hand, is a function of signal to noise ratio and only indicates the above-defined minimum optical power level.
The prior art approach to designing an optical receiver is to serially connect an optical detector to the input of a low-noise, fixed gain amplifier. An automatic gain circuit (AGC) is then coupled to the output of the low-noise amplifier to maintain the receiver output at the desired amplitude. While this approach works satisfactorily, the resulting optical receiver often has insufficient dynamic range. In telecommunications applications, for example, the range of optical power levels causes overloading of the AGC circuit resulting in a distorted receiver output signal. In optical telecommunications systems wherein the wavelength of the optical source is less than 1 micron (.mu.m), hereinafter referred to as "short" wavelength systems, the problem of overloading has been solved through the use of an AGC loop to control the gain of the optical detector. (See, for example, a publication by T. L. Maione et al entitled "Atlanta Fiber System Experiment: Practical 45-Mbs Regenerator for Lightwave Transmission," Bell System Technical Journal, Vol. 57, No. 6, July-August 1978, pages 1837-1856). This technique, however, cannot be applied to systems wherein the wavelength of the optical source is greater than 1 micron, hereinafter referred to as "long" wavelength systems, since the applicable detectors have unity gain. The problem of insufficient dynamic range in receivers for "long" wavelength systems is, therefore, as yet unresolved. Moreover, the solution to this problem is of significant import since "long" wavelength systems exhibit lower signal losses per unit distance than comparable "short" wavelength optical transmission systems.